As the trailers indicated, Valerian is visually spectacular. Stunning even. The colors, sets, props, costumes, makeup, and world-building are all well above par. The opening credits scene, which succinctly shows the story of the space city, was powerfully and imaginatively shown. Throughout the film, new locales are expertly unveiled, and action scenes unfold like eye candy. I didn't notice music, or lack thereof, which is in my opinion perfect. (Exception: I noticed the opening music track, but in a good way.)
But very early, as soon as we're introduced to the two main characters, I felt something was off. I soon decided it was casting. Dane DeHaan (whom I loved in "Two Lovers And A Bear") simply was not believable as a super-soldier nor as a man with a string of romantic conquests. He both appeared and presented as an adolescent rather than a battle-hardened major. Cara Delevingne played better, but often went dead-faced when it wasn't appropriate (yet it served her well in "Suicide Squad").
Both characters had lines that didn't seem to fit what we'd been told about them, and very often didn't fit with the lines that came immediately before and after them, almost as if someone cut the script into individual lines and shuffled them. The character arcs seemed non-existent most of the time, but then would jump at the oddest moments, giving an unnatural feel to these two main characters.
The script had other problems as well. For example, quite a bit of dialog was used to describe what was visually apparent -- almost as if whoever was in charge of the lines did not know what would be visible on screen, or vice versa. It came across as condescending to watch a visual explanation of something and then have a character explain it verbally too.
One final complaint: The film had several instances of a common problem with both sci-fi and superhero movies: introducing a power or technology and then either forgetting it exists or immediately introducing something else to countermand it. Valerian seemed to introduce a hundred futuristic devices, discoveries, and technologies, all of which would be interesting to explore, but they were mostly ignored or simply used as a prop for a single scene. (Besson's "Fifth Element" had this irritating issue as well.) So many of them could have made this movie incredible if they'd been introduced with the intent of bringing them back later as part of the climax; instead they became throwaway comedic devices.